Semiconductor laser with reduced heat loss

ABSTRACT

Disclosed is a semiconductor laser. The semiconductor laser includes a semiconductor chip that includes an active layer and emits radiation in a main radiating direction. The active layer is structured in a direction perpendicular to the main radiating direction to reduce heating of the semiconductor chip by spontaneously emitted radiation. The active layer includes a region provided for optical pumping by a pump radiation source. The optically pumped region of the active layer is surrounded by a region having, in a direction perpendicular to the main radiating direction, a periodic structure that forms a photonic crystal in which radiation having the emission wavelength is not capable of propagation.

RELATED APPLICATIONS

This patent application claims the priority of German Patent Application103 39 980.1-54 filed Aug. 29, 2003, and is a Division of U.S. patentapplication Ser. No. 10/926,465 filed Aug. 25, 2004 now U.S. Pat. No.7,356,062, the disclosures of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The invention relates to a semiconductor laser having a semiconductorchip which contains an active layer and emits radiation in a mainradiating direction.

BACKGROUND OF THE INVENTION

The efficiency and output power of semiconductor lasers are cruciallylimited by the heat arising as a result of radiative and non-radiativerecombinations of the injected charge carriers.

In order to improve the heat dissipation, optoelectronic components areoften provided with a heat sink. In this case, the temperature increaseΔT in the active zone is given by ΔT=R_(th)·P_(v), where R_(th) is thethermal resistance between the active zone and the heat sink and P_(v)is the power loss. A temperature increase generally brings about a loweramplification in the case of a semiconductor laser. In order to reducesuch a temperature increase which is disadvantageous for the efficiencyof the semiconductor laser, it is advantageous to reduce the thermalresistance R_(th) or the power loss P_(v).

By way of example, Kusznetsov et. Al., IEEE Journ. Sel. Topics inQuantum Electronics 5 (3), 561 (1999) discloses improving the thermallinking of the heat sink to the active zone by removal of a substratethat is typically arranged between the active zone and the heat sink.The thermal resistance R_(th) and the temperature increase ΔT arethereby reduced.

The power loss P_(v) arising within the active layer is cruciallydetermined by the non-radiative recombination of charge carriers bymeans of defects or by means of Auger processes, and by radiativerecombinations of charge carriers by spontaneous emission. A highrecombination rate leads to a short lifetime of the induced chargecarriers and thus to a high laser threshold, a low efficiency and outputpower. It is desirable, therefore, to minimize these types ofrecombination of charge carriers. The non-radiative recombination ofcharge carriers by means of defects can be influenced by the quality ofthe epitaxy. By contrast, the non-radiative recombination of chargecarriers by means of Auger processes is difficult to influence.

The spontaneous emission of radiation can be influenced by structures ofthe order of magnitude of the light wavelength, for example bymicroresonators or photonic crystals. A detailed presentation of themode of operation and the methods for production of photonic crystals iscontained in the document T. F. Krauss, R. M. De La Rue, Prog. Quant.Electr. 23 (1999) 51-96, the content of which is hereby incorporated byreference.

The influencing of the spontaneous emission by a microresonator isdisclosed in the document Y. Hanamaki, H. Akiyama, Y. Shiraki, Semicond.Sci. Technol. 14 (1999) 797-803. U.S. Pat. No. 5,420,880 describesreducing the laser threshold of a surface-emitting semiconductor laserby means of reducing the coupling-out of spontaneously emittedradiation.

Furthermore, WO 98/43329 discloses a vertically emitting laser in whichthe radiation emitted spontaneously or in stimulated fashion from afirst volume region of an active medium in a transverse direction isutilized for pumping a second volume region surrounding the first volumeregion.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a semiconductor laserhaving a comparatively small degree of heating during operation.

One aspect of the invention is directed to a semiconductor laser thatincludes a semiconductor chip that includes an active layer and emitsradiation in a main radiating direction. The active layer is structuredin a direction perpendicular to the main radiating direction to reduceheating of the semiconductor chip by spontaneously emitted radiation.The active layer includes a region provided for optical pumping by apump radiation source. The optically pumped region of the active layeris surrounded by a region having, in a direction perpendicular to themain radiating direction, a periodic structure that forms a photoniccrystal in which radiation having the emission wavelength is not capableof propagation.

In one embodiment, the semiconductor laser is a disc laser, and the disclaser has an external resonator. In another embodiment, the active layerhas the form of a mesa. In still another embodiment,

In yet another embodiment, the active layer has the form of a mesa, andthe width of the mesa in a direction perpendicular to the main radiatingdirection of the semiconductor laser is approximately as large as thewidth of the optically pumped region.

In another embodiment, the periodic structure is formed by alattice-type arrangement of cutouts, and the cutouts are filled by amaterial whose refractive index differs from that of the active layer.

In still another embodiment, the laser contains a heat sink, and nosubstrate is contained between the heat sink and the active layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagrammatic illustration of a cross section through afirst example of a first embodiment of a semiconductor laser accordingto the invention,

FIG. 2 shows a diagrammatic illustration of a cross section through asecond example of the first embodiment of a semiconductor laseraccording to the invention,

FIG. 3 a shows a diagrammatic illustration of a cross section through asecond example of a second exemplary embodiment of a semiconductor laseraccording to the invention,

FIG. 3 b shows a diagrammatic sectional view of the active layer of thethird exemplary embodiment from above, and

FIG. 4 shows a simulation of the output power of a semiconductor laseras a function of the pump power, the intensity and the coupling-out ofthe spontaneously emitted radiation having been varied.

FIG. 5 shows a first example of the second embodiment of a semiconductorlaser according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Identical or identically acting elements are provided with the samereference symbols in the figures.

The semiconductor laser illustrated in FIG. 1 is a surface-emittingoptically pumped semiconductor laser with an external resonator(VECSEL). The semiconductor laser has a semiconductor chip 1 containinga substrate 2, a reflector 3, and an active layer 5. The reflector 3,which is preferably a Bragg reflector, forms, together with an externalreflector 4, the resonator of the semiconductor laser. The semiconductorlaser emits radiation in the main radiating direction 6.

The active layer 5 may, for example, have a quantum well structure andcontain in particular III-V compound semiconductor materials. A region 7of the active layer 5 is optically pumped by a pump radiation source 8.The active layer 5 has the form of a mesa. In a direction perpendicularto the main radiating direction 6 of the semiconductor laser, the mesahas a smaller cross section than the rest of the semiconductor chip 1.The mesa structure is produced for example by means of aphotolithographic patterning and a subsequent etching process.

Radiation 10 that is spontaneously emitted within the region 7, intowhich the pump radiation source 8 radiates pump radiation 9, andpropagates in the direction of the side walls 11 of the mesa reaches theside walls 11 after a comparatively short path through regions 12 of theactive layer 5 that are not optically pumped and therefore absorb.Within the optically pumped region 7, by contrast, spontaneously emittedradiation 10 can propagate in a manner virtually free of absorption,since a population inversion is present there on account of the pumpprocess. The shape of the active layer 5 as a mesa structure reduces thenon-inverted regions 12 in a direction perpendicular to the mainradiating direction, so that the absorption of spontaneously emittedradiation is advantageously reduced in these regions 12.

Particularly preferably, the mesa is approximately as wide as theoptically pumped region 7 in a direction perpendicular to the mainradiating direction 6. The width of the non-inverted regions 12 that canabsorb spontaneously emitted radiation 10 is therefore reduced to aminimum.

In order to further improve the coupling-out of spontaneously emittedradiation 10 from the semiconductor chip 1, the side walls 11 of themesa are advantageously provided with an antireflective layer 13. Thisreduces the back-reflection on account of the difference in refractiveindex of the active layer 5 with respect to the surroundings.

The reduced absorption of spontaneously emitted radiation 10 reduces thethermal loading on the semiconductor laser. In order to better dissipateheat loss that occurs nevertheless, the semiconductor laser mayfurthermore contain a heat sink 14. By way of example, the semiconductorchip 1 may be mounted onto the heat sink 14 by the rear side of thesemiconductor substrate 2. The thermal resistance between the activelayer 5 and the heat sink 14 may advantageously be reduced by removingthe substrate 2. Such a semiconductor laser is illustrated in FIG. 2.

The second example of the first embodiment of a semiconductor laseraccording to the invention as illustrated in FIG. 2 differs from thatillustrated in FIG. 1 furthermore by virtue of the fact that the sidewalls 15 of the mesa run obliquely with respect to the main radiatingdirection 6 of the semiconductor laser. The oblique side walls 15advantageously do not form a resonator and thus prevent a build-up ofoscillations of the laser in a direction perpendicular to the mainradiating direction 6. In this exemplary embodiment, too, the side walls15 of the mesa are preferably provided with an antireflective layer 13.

The second embodiment of the invention, the aim of which is actually toreduce the production of the spontaneous emission, may be realized forexample in the example illustrated in FIG. 5 by virtue of the fact thatthe side walls 11 of the mesa form a resonator which reduces thespontaneous emission in a direction perpendicular to the main radiatingdirection. In this embodiment of the invention, it may even beadvantageous to provide the side walls 11 of the mesa with areflection-increasing layer 16.

A second example of the second embodiment of the invention isillustrated in FIGS. 3 a and 3 b. Apart from the configuration of theactive layer 5, the semiconductor laser illustrated in FIG. 3 acorresponds to the semiconductor laser illustrated in FIG. 1. The activelayer 5 is patterned in such a way that the optically pumped region 7,in a direction perpendicular to the main radiating direction 6 of thelaser, is surrounded by a region 17 having a lattice structure thatforms a photonic crystal in which radiation having the emissionwavelength is not capable of propagation.

The lattice structure is realized for example by a region 17 of theactive layer 5 that surrounds the optically pumped region 7 beingprovided with a periodic arrangement of cutouts 18. This is illustratedfrom above in the sectional view of the active layer 5 as illustrated inFIG. 3 b. The cutouts 18 may also be filled by a material having arefractive index that differs from the refractive index of thesurrounding semiconductor material.

The results of a simulation calculation as shown in FIG. 4 once againillustrate the fundamental idea of the two embodiments of the invention.The illustration shows simulations of the output power P_(L) of asemiconductor laser as a function of the power of the pump radiationsource P_(P).

In the case of the simulation of the curve 19 illustrated with squaresymbols, it was assumed that spontaneously emitted radiation arising inthe semiconductor laser is not coupled out. Given a pump power of aboveapproximately 17 W, the semiconductor laser exhibits a severe dip in theoutput power on account of thermal overheating.

The curve 20 illustrated with circular symbols was simulated under theassumption that 50% of the spontaneously emitted radiation is coupledout from the semiconductor chip. This improved coupling-out ofspontaneously emitted radiation corresponds to the basic idea of thefirst embodiment of the invention. The curve 20 illustrates that theoutput power does not dip until at a higher pump power and, therefore,it is also possible to achieve a higher output power.

The curve 21 illustrated with triangular symbols illustrates theprinciple of the second embodiment of the invention. In this simulation,it was assumed that although spontaneously emitted radiation is notcoupled out from the semiconductor chip, the spontaneously emittedradiation is reduced overall by 50%. In this case, too, it is possibleto achieve higher output powers than in the case of the curve 19. In thecase of the simulated curve 21, given the same pump power, the outputpower of the semiconductor laser is greater than in the case of thecurve 20 since the laser threshold is also lowered overall as a resultof the reduction of the spontaneous emission. Therefore, the laseractivity also already commences at a lower pump power.

The invention is not restricted by the description on the basis of theexemplary embodiments. Rather, the invention encompasses any new featureand also any combination of features, which in particular comprises anycombination of features in the patent claims, even if this feature orthis combination itself is not explicitly specified in the patent claimsor exemplary embodiments.

1. A semiconductor laser, comprising: a semiconductor chip comprising anactive layer and emitting radiation in a main radiating direction;wherein the active layer is structured in a direction perpendicular tothe main radiating direction to reduce heating of the semiconductor chipby spontaneously emitted radiation; wherein the active layer includes aregion provided for optical pumping by a pump radiation source; andwherein the optically pumped region of the active layer is surrounded bya region having, in a direction perpendicular to the main radiatingdirection, a periodic structure that forms a photonic crystal in whichradiation having the emission wavelength is not capable of propagation.2. The semiconductor laser according to claim 1, wherein thesemiconductor laser is a disc laser.
 3. The semiconductor laseraccording to claim 2, wherein the disc laser has an external resonator.4. The semiconductor laser according to claim 1, wherein the activelayer has the form of a mesa.
 5. The semiconductor laser according toclaim 1, wherein the active layer has the form of a mesa, and the widthof the mesa in a direction perpendicular to the main radiating directionof the semiconductor laser is approximately as large as the width of theoptically pumped region.
 6. The semiconductor laser according to claim1, wherein the periodic structure is formed by a lattice-typearrangement of cutouts.
 7. The semiconductor laser according to claim 6,wherein the cutouts are filled by a material having refractive indexthat differs from that of the active layer.
 8. The semiconductor laseraccording to claim 1, wherein the laser contains a heat sink.
 9. Thesemiconductor laser according to claim 8, wherein no substrate iscontained between the heat sink and the active layer.